1. Field of the Invention
The present invention relates to a liquid crystal display device. The invention is particularly suitable for a liquid crystal display device to which the orientation treatment is applied to an orientation film with the radiation of light.
2. Description of the Related Arts
In the manufacture of a liquid crystal display device, as the general treatment of a liquid crystal orientation control layer, treatment which is generally-called “rubbing” in which an organic film which is formed on a substrate and is made of polyimide is rubbed with a cloth is performed. Such rubbing treatment gives rise to drawbacks such as the contamination attributed to the generation of dust at the time of rubbing, the occurrence of electrostatic damages attributed to a friction with respect to a transparent substrate on which TFT-type elements are mounted, and the lowering of a manufacturing yield rate attributed to the occurrence of electrostatic damages. Accordingly, there has been a demand for non-contact-type liquid crystal orientation techniques, and as one of such techniques, the light orientation treatment has been known (U.S. Pat. No. 5,604,615, referred to as patent document 1 hereinafter).
The technique disclosed in patent document 1 is a technique in which polarized ultraviolet rays are radiated to an organic orientation film formed on a transparent substrate thus generating a chemical reaction to molecules which constitute the organic orientation film corresponding to the polarization direction of the ultraviolet rays whereby the directivity and a pretilt angle of the liquid crystal orientation are imparted to the organic orientation film. Accordingly, due to such a technique, the contamination attributed to the generation of dusts at the time of performing the rubbing, and the generation of electrostatic damages to a TFT type element mounting substrate can be prevented and, at the same time, the lowering of a manufacturing yield rate can be also prevented.
Further, according to the currently available liquid crystal display device, for broadening a viewing angle of the liquid crystal display device or for compensating for a residual phase difference of a liquid crystal cell used in the liquid crystal display device which sandwiches a liquid crystal layer between two substrates, a layer which has the azimuth-angle retardation and is referred to as a phase difference plate is sandwiched between the liquid crystal cell which sandwiches the liquid crystal layer between a pair of substrates and polarizers of the liquid crystal cell. That is, the polarizer, the phase difference plate, the liquid crystal cell, the phase difference plate and the polarizer are stacked in this order from a light incident side, for example.
For example, in a TN-type liquid crystal cell in which the liquid crystal molecules are twist-oriented at an angle of approximately 90°, as described in Japanese Patent Laid-open Hei6-167707 (patent document 2), it is considered possible to suppress the inversion of color tone even when a viewing angle is changed. Further, in a normally-white homogeneous-oriented liquid crystal cell in which liquid crystal molecules are arranged in parallel, as described in Japanese Patent Laid-open No. 255347/2003 (patent document 3), a phase difference plate is used for compensating for a residual phase difference in the inside of the liquid crystal cell. Even in a so-called VA-type liquid crystal cell in which liquid crystal molecules are arranged perpendicular to a plate of the liquid crystal cell, as described in Japanese Patent Laid-open Hei11-2842 (patent document 4), it may be possible to adopt the structure which allows the liquid crystal molecules to slightly twist while tilting in an ON state by applying slight rubbing by focusing on the viewing angle characteristic. In this case, the liquid crystal molecules are not arranged completely perpendicular to the liquid crystal cell and hence, the residual phase difference is generated in the inside of the liquid crystal cell. However, this residual phase difference is not so large and hence, the influence of the residual phase difference is smaller than the corresponding influence in the homogeneous orientation and the acquisition of the phase plate having the small phase difference is difficult whereby it is considered that the phase difference is not compensated.